In recent years, considerable efforts have been made by the coatings industry to develop coating formulations containing little or no volatile organic compound (VOC) content. Regulations to limit the amount of VOC content of industrial coatings have encouraged research and development to explore new technologies directed at reducing solvent emissions from industrial solvent-based coatings operations used to coat such products as automotive parts, appliances, general metal products, furniture, and the like. However, while the move to reduced organic solvent-based compositions brings health and safety benefits, these lower VOC content coating compositions must still meet or exceed the performance standards expected from solvent-based compositions.
Alkyd resins are one of the most common binders used for ambient-cure, solvent-based coatings. The resistance properties of traditional solvent-borne alkyd resins are developed via autooxidative crosslinking of the alkyd film. Crosslinking occurs when the activated methylene groups in the unsaturated fatty acids or oils of the alkyd are oxidized in air to give hydroperoxides which subsequently decompose to generate free radicals, resulting in oxidative crosslinking. This oxidative crosslinking process is commonly accelerated by adding driers, such as, for example, various salts of cobalt, zirconium, calcium, and manganese. However, while alkyd resins have shown, and continue to show, promise, they have relatively slow “dry” and/or cure times, particularly at ambient temperatures. Various modifications have been made to alkyd resins to address such concerns.
One such attempt includes polymerization of an alkyd resin with a vinyl compound, such as styrene or methyl methacrylate, via a free-radical reaction, to produce a vinyl-alkyd copolymer or a vinyl alkyd. Vinyl alkyd resins generally have a higher molecular weight and a higher Tg, producing coatings with reduced tack-free time (solvent evaporation). However, the through-dry time (oxidation of the film) of such coatings is longer due to the decreased degree of unsaturation in the alkyd as a result of copolymerization with the vinyl compound. This problem is described in further detail in Resins for Surface Coatings, Vol. 1, pp. 181 et seq., ed. by P. K. T. Oldring and G. Hayward, SITA Technology, London, UK, 1987, which is incorporated herein by reference. An additional drawback is that paint formulations containing vinyl alkyd resins require greater amounts of solvent, due to the increased molecular weight and Tg of the vinyl alkyd.
Conventional long oil alkyds are used throughout the industry as the main binder in high gloss architectural trim enamels. Typical alkyds are made by reacting soybean oil with pentaerythritol (PE) via alcoholysis, and then reacting the reaction product in a second stage with phthalic anhydride (PAN). The result is a long oil alkyd with good through-dry. The use of pentaerythritol provides an alkyd with high branching and number average molecular weight (Mn), a light color, improved yellowing resistance, and low cost.
High solids alkyds have been developed for use in high gloss architectural trim enamels having a VOC content of less than 250 g/L. Reduction in viscosity in these resins is achieved by lowering the amount of PE, which results in less branching and a lower Mn. One such alkyd is Eastman's Duramac HS 5816, which is made from sunflower oil reacted with pentaerythritol (PE) via alcoholysis, followed by reacting the reaction product with a fatty acid, and then phthalic anhydride. The result is a long oil alkyd having reasonable through dry, light color, and a reasonable cost, but having less satisfactory yellowing resistance.
Thus, there is a trade-off between through dry and yellowing. Less yellowing is observed with less conjugated fatty acids and oils. Another drawback of such systems is that typical high solids alkyds result in paints that exhibit stringiness or ropiness (brush drag and high ICI viscosity).
JP 48085628 describes a modified alkyd resin using glycidyl acrylate, glycidyl methacrylate, or its derivative. Drying oil-modified alkyd resins having carboxyl groups and an oil length of 20-80 are treated with glycidyl acrylate, glycidyl methacrylate, or its derivative, in the presence of a polymerization inhibitor. The resulting resin is mixed with a photosensitizer or photoinitiator to give a coating composition which hardens with UV irradiation. However, the resin compositions disclosed are not suitable for ambient oxidative cure, high-solids coating applications.
PCT Appl. Publ. No. WO 01/00741, incorporated herein by reference, discloses an ambient oxidative cure composition based on an acrylate-functionalized alkyd resin. The acrylate-functionalized alkyd resin is prepared by reacting an alkyd resin having an acid number of from 0 to about 10 mg KOH/g with an acid anhydride, such as trimellitic anhydride, to produce a carboxyl-functional alkyd resin, and reacting the carboxyl-functional alkyd resin with a glycidyl acrylate, to produce an acrylate-functionalized alkyd resin. These acrylate-functionalized alkyd resins are then used in coating compositions in which conventional drier mixtures are used, providing a calcium content in the coating composition of from 0.05% to 0.1% metal based on resin solids, and a cobalt content in the coating composition of from 0.02% to 0.15% metal based on resin solids.
U.S. patent application Ser. No. 09/596,269, filed Jun. 16, 2000 and incorporated herein by reference, describes an acrylate-functional alkyd coating composition comprising an acrylate-functionalized alkyd resin, at least one drier, and water or an organic solvent. The acrylate-functionalized alkyd resin described is the reaction product of an alkyd resin and a glycidyl acrylate such as glycidyl methacrylate, the glycidyl moiety of the glycidyl acrylate being the reactive moiety that functionalizes the alkyd resin. The resulting reaction product contains pendant reactive acrylate moieties.
The compositions described in U.S. patent application Ser. No. 09/596,269 exhibit improved dry time, on the order of 3 hours or more set to touch time, making them suitable for fast-dry, ambient-cure coating applications. These dry times are achieved with conventional drier packages, providing a calcium content in the coating composition of about 0.05% to 0.1% metal based on resin solids, and a cobalt content in the coating composition of from 0.02% to 0.15% metal based on resin solids.
Although these compositions are an advance in the art, there remains a need in the market to further improve the surface dry time of low VOC alkyd paint to a very low level, such as 2 hour set to touch, 4 hour tack free, and 8 hour though-dry, so that the painted surface can be handled in a short time, and so that the surface will exhibit less dust pick up, resulting in cost savings and an improved appearance. This has been very difficult to achieve for low VOC alkyds, because these low molecular weight alkyds characteristically exhibit a longer dry time, during which the molecular weight of the alkyd is built up via crosslinking. Although the acrylate-functional alkyd resins described in U.S. patent application Ser. No. 09/596,269 have improved dry time using conventional drier packages, the set to touch and surface dry time achieved using conventional drier packages are still not fast enough to satisfy some demanding applications.